The most widely used stylus type electrostatic recorders today apply a charge to a recording medium through a pair of coincident voltage pulses applied to opposite sides of the recording medium by means of a plurality of stylus electrodes and backup or counter electrodes. The backup electrode arrangement usually takes the form of a plurality of U-shaped shoes fabricated from a metal block having an elongated channel. The U-shaped backup electrode arrangement, as compared, for example, to the empolyment of a flat plate backup electrode arrangement, provides reduced wear on the stylus electrodes and its head assembly while also protecting to some extent the stylus electrodes from dielectric breakdown.
Head wear in this older arrangement was reduced because the spaced but parallel lips of the U-shaped backup electrode provided minimal contact with the recording medium, permitting the medium to freely adjust to accomodate for small physical fluctuations in the profile of the stylus head. Complete dielectric breakdown was minimized because the effective resistance of the recording medium in contact between the backup electrode lips and the stylus electrodes provided sufficient protection against pinhole shorts in the recording medium which would otherwise cause machine damage to the stylus electrodes and recorder circuitry.
However, the U-shaped backup electrode arrangement having been successfully employed in the past has not been so employed without several disadvantages. To begin with, their fabrication entails detail fabrication and expensive production costs. The alignment of the backup electrode segments relative to the stylus head during installation in the recorder is time consuming.
The U-shaped backup electrode arrangement provides an recording medium increment in the charging path between the recorder electrodes, the impedance value of which is variable. To begin with, the printout quality of these electrostatic recorders depends strongly on the conductivity nature of the paper base or substrate of the recording medium, which varies along the length of the medium, as well as, from roll to roll of the recording medium. The variances of conductivity is usually the result of the paper characteristic itself and may be contributed to by changes in relative humidity. The changes in conductivity affects the charging characteristics in the charging path between the backup electrodes and the stylus electrode array, subjecting solid black areas during development, to striations in the direction of recording medium travel. These are undesirable, as they cause a blurred appearance of the overall developed image.
Lastly, because there is an effective recording medium increment included in the charging path between the stylus electrode array and the lips of the U-shaped backup electrodes, parallel charging paths are created which increase the charging path impedance thereby necessitating longer write times. Longer write times (longer charge depositing time due principally to a higher RC time constant in the charging path) means a slower printing process.
One manner of improving printer speed is to decrease this impedance in the charging path while not subjecting the recorder to an increased likelihood of pinhole shorting through the recording medium while further providing efficient recording medium alignment for faster movement of the recording medium through the recording station and the development station.
U.S. Pat. No. 3,693,181 suggests the reduction of the impedance in the charging path, as well as, casing the alignment problem by providing a backup electrode arrangement having a concaved conductive surface overlying a resilient spongy layer. However, this conductive type backup electrode presents the danger of recorder damage, particularly to the stylus electrodes, due to pinhole shorts through the recording medium. A pinhole short across the charging path will blow out or burn up the electrode nibs of one or more recording stylus in the head assembly rendering the recording head useless for future use. Also, printout quality will still be affected by conductivity changes of the moving recording medium.